Many children with specific language impairment (SLI) show an especially serious limitation in the use of grammatical morphemes. The problematic morphemes include verb inflections such as -s (as in "She walks to work"), and function words such as auxiliary verbs (as in "Chris is talking on the phone"). The problem can be longstanding, often carrying into the school years. The purpose of this project is to explore the possible bases of these grammatical morpheme limitations. To accomplish this goal, an experimental treatment paradigm will be employed. Each of the alternative explanations of grammatical morpheme limitations in children with SLI provides a reasonable account of the grammatical morphemes commonly studied. However, these accounts differ in their predictions across a wider range of grammatical morphemes. By monitoring the children's production and comprehension of different sets of grammatical morphemes and providing the children with intensive treatment on one set, the pattern of gains seen on the untreated sets should reveal the nature of the relationship among the morphemes. Because no two accounts make the same predictions for all morphemes, the resulting pattern of data should lend support to some accounts and argue against others. The treatment design to be employed should also provide important benefits of a clinical nature. The large number of children with SLI participating and the wide range of grammatical morphemes to be studied should provide important information about both the efficacy of treatment and the generalization patterns that might be expected from this treatment.